▼ We have fabricated and investigated the electrical and
magnetic behavior of polycrystalline and epitaxial CrO2
nanostructures, grown using selective-area growth technique.
Magnetic domain structures were studied by magnetic force
microscopy, and in-plane, lamellar domain structure with fragmented
walls aligned along the magnetic easy axis direction have been
observed, indicating a large magnetocrystalline anisotropy in
epitaxial CrO2 nanostructures. Low-temperature transport
measurements on nanowires have shown that the dc resistivity of
polycrystalline CrO2 wires is strongly dependent on the linewidth.
Below a critical temperature, a transition from a positive to a
negative temperature coefficient of resistivity have been observed,
which we attribute to a competition between the scattering of the
conduction electrons inside the grains and scattering across the
grain boundaries. Using a model based on grain boundary scattering,
we estimate a mean transmission probability through the grain
boundaries to be on the order of 10?1. Furthermore,
magnetoresistance (MR) measurement indicates that the MR behavior
of polycrystalline wires is dominated by the shape anisotropy;
however, for epitaxial wires, both the shape and magnetocrystalline
anisotropy play important roles, and the resulting MR properties
are found to be closely related to the orientation of the wire
axis. By studying the MR curves, we inferred the internal domain
structures in various single crystal CrO2 wires and found that the
spin-dependent transport is much stronger across a grain boundary
than a domain wall. We have also studied the magnetotransport
properties of CrO2 nanocontacts. Manipulating the domain walls
using a large dc current in the contact area yields a
magnetoresistance of up to 25%, which is the largest ever seen in a
single ferromagnetic film. The single domain-wall-resistance is
determined to be three orders of magnitude larger than that of
conventional 3d ferromagnets, as a result of the material's
half-metallicity. We have measured DWR and the spin-torque effect
along different crystallographic axes and at varying temperatures.
Finally, we present the results of a theoretical analysis of this
system, based on its half-metallic character and on the intrinsic
magnetic behavior of CrO2.
Advisors/Committee Members: Xiao, Gang (director), Sun, Shouheng (reader), Mitrovic, Vesna (reader).

► The quantum Hall effect is a marvelous thing, a microcosm of all that is strange and beautiful and quantum. The fractional quantum Hall effect supports…
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▼ The quantum Hall effect is a marvelous thing, a
microcosm of all that is strange and beautiful and quantum. The
fractional quantum Hall effect supports quasiparticles of
fractional charge and exotic statistics. These systems are not just
a theorist's fancy and are measurable in lab. Even for properties
that are still waiting for conclusive experimental confirmation, we
have strong theoretical reasons to assume their existence, giving
something for experimentalists to strive for as well. After nearly
40 years, the field isn't closed and can still yield surprises. In
this thesis, a number of novel probes of the nature of the
topological order existing in these systems are discussed, in
particular focussing on the mystery of the ν=5/2 filling factor.
Because of the bulk gap, low energy physics in the quantum Hall
effect is confined to the edges of the 2D electron liquid. The
velocities of edge modes are key parameters of edge physics. They
were determined in several quantum Hall systems from time-resolved
measurements and high-frequency ac transport. A way is proposed to
extract edge velocities from dc transport in a point contact
geometry defined by narrow gates. The width of the gates assumes
two different sizes at small and large distances from the point
contact. The Coulomb interaction across the gates depends on the
gate width and affects the conductance of the contact. The
conductance exhibits two different temperature dependencies at high
and low temperatures. The transition between the two regimes is
determined by the edge velocity. An interesting feature of the
low-temperature I-V curve is current oscillations as a function of
the voltage. The oscillations emerge due to charge reflection from
the interface of the regions defined by the narrow and wide
sections of the gates. Numerical results suggest that the quantum
Hall effect at ν=5/2 is described by the Pfaffian or
anti-Pfaffian state in the absence of disorder and Landau level
mixing. Those states are incompatible with the observed transport
properties of GaAs heterostructures, where disorder and Landau
level mixing are strong. We show that the recent proposal of a
PH-Pfaffian topological order by Son is consistent with all
experiments. The absence of the particle-hole symmetry at ν=5/2
is not an obstacle to the existence of the PH-Pfaffian order since
the order is robust to symmetry breaking.
Advisors/Committee Members: Feldman, Dmitri (Advisor), Mitrovic, Vesna (Reader), Valles, James (Reader).

▼ Study of the combined effects of strong electronic
correlations with spin-orbit coupling (SOC) represents a central
issue in quantum materials research. Predicting emergent properties
represents a huge theoretical problems as presence of SOC implies
that the spin is not a good quantum number. A multitude of exotic
quantum phases are predicted to emerge even in materials with
simple cubic crystal structure such as Ba2NaOsO6. Experimental
tests by local probes, such as Nuclear magnetic resonance (NMR),
are highly sought for. In this thesis, we first present the
temperature evolution of the 23Na NMR spectrum. It clearly reveals
a geometrical distortion, preceding a formation of the long-range
ordered (LRO) magnetism. We discovered from the angular dependence
measurement that a cubic to orthorhombic structural phase
transition occurs, induced by elongation/compression of the oxygen
octahedra, and only one structurally equivalent environment exists
for 23Na nuclei. Furthermore, from a lattice sum simulation we
found that the low temperature LRO state is the canted
two-sublattice ferromagnetic (FM) state, which gives rise to the
magnetic splitting in NMR spectrum. This is the first direct
observation of such exotic magnetic order in 5d transition-metal
systems. Such state is predicted to occur due to multipolar
spin-spin interactions on the frustrated fcc lattice. We provide
evidence for the presence of complicated orbital ordering. Lastly,
we studied the magnetic order within the intermediate transition
region, and the possible existence of spin nematic order is
discussed based on second moment analysis. This work paves the way
for future NMR study of highly frustrated systems with the strong
SOC and electron correlations, such as Iridates.
Advisors/Committee Members: Mitrovic, Vesna (Director), Valles, James (Reader), Marston, John (Reader).

► Experiments have established the existence of a Bose Insulator phase close to the two dimensional Superconductor-Insulator transition. Here, current is carried by localized Cooper Pairs,…
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▼ Experiments have established the existence of a Bose
Insulator phase close to the two dimensional
Superconductor-Insulator transition. Here, current is carried by
localized Cooper Pairs, which are usually responsible for
superconductivity. Several systems show a unified picture of this
phase, where Cooper Pairs themselves behave as individual bosons.
However, the limits of the region of phase space that it occupies
are poorly understood, and there has been limited work examining
whether or not pairs survive in the limit of strong disorder. To
address this question, we conducted experiments with films grown on
nanostructured templates made of Anodic Aluminum Oxide (AAO). We
are able to study the influence of both material and topographical
properties of thin films on localization in both the bosonic and
fermionic regimes. I will detail two studies examining the breadth
of these regimes. In the first, finite size effects on systems of
weakly coupled superconducting islands are studied by growing films
on AAO substrates with small hole spacings. Some experiments show
evidence of a substantial shrinking of the bose insulator phase,
while others are more ambiguous. We speculate that the degree of
inhomogeneity present in the substrates leads to more robust
pairing in some films, enhancing the bose insulator phase relative
to others. In the second approach, I study scaling of the high
temperature resistance, where Cooper pairs are not expected. Since
scaling is observed across many fermionic systems, deviations from
it can be used as a probe to detect Cooper pairs. Our results show
bosons present in the strongly localized regime, where transport is
fermionic. I also detail numerical studies of the influence of
inhomogeneity on scaling. The simulations show that uniform
disorder does not substantially impact the results of scaling, but
correlated inhomogeneities can have a drastic impact on the
experimental results.
Advisors/Committee Members: Valles, James (Advisor), Feldman, Dmitri (Reader), Mitrovic, Vesna (Reader).

▼ Anodized aluminum oxide (AAO) is an interesting
material, occupying a unique space – already used incredibly
widely in the world, yet also commonly used for nanoscience
research. However, the wealth of scientific research using it has
yet to make it to the real world. Similarly, many nanoscience
fields that have produced incredible results on a small scale
haven’t found practical use. In this dissertation, we investigate
scalable applications of AAO in two fields, and then look at their
potential intersection. First, we look at its use as a scalable
structural coloration platform, employing a Fabry-Perot optical
cavity structure that has seen a research resurgence in recent
years. Structural coloration presents the potential for a more
tunable, durable, and environmentally friendly coloration system
than traditional methods, though it can also be used in concert
with them. We present tools to analyze the full capabilities of
this structure for its future use in structural coloration as a
“color by design” platform and verify them with experimental probes
of fabricated samples. Further, we look to mitigate assumed
limitations of this system, such as sensitivity to fabrication
parameters and angular dependence. Then, we present another
achievement of AAO as a scalable platform: the dielectric of a
Ag-AAO-Al resistive switching (RS) system used for optical
rectification (OR). RS is a well researched field, but almost
entirely in the electronic context, being the first realization of
the memristor. OR is a promising field with potential for energy
harvesting and ultrafast light sensing that has seen a resurgence
in recent years, but all published methods are either highly
unrepeatable or rely on advanced nanofabrication techniques. We
demonstrate an intersection of the RS field, used to create a
unique OR platform that offers scalability and tunability beyond
any that have been reported. We investigate the electronic effects,
temperature dependence, and then present possible OR effects.
Lastly, we present some potential combinations of these two
projects.
Advisors/Committee Members: Xu, Jimmy (Advisor), Valles, James (Reader), Mitrovic, Vesna (Reader).

► The experimental realization of a two dimensional quantum spin liquid has been eagerly sought since first proposed by Anderson in 1973. Recently, Cs2CuCl4, a frustrated…
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▼ The experimental realization of a two dimensional
quantum spin liquid has been eagerly sought since first proposed by
Anderson in 1973. Recently, Cs2CuCl4, a frustrated 2D quantum
antiferromagnet (QAF), has shown potential in this regard
triggering immense interest, both experimentally and theoretically.
Amongst these studies, different scenarios on the nature of the
spin liquid phase were proposed and the presence of exotic magnetic
phases in the ground state was noticed. To this date, many of these
phases are still obscured and the nature of the spin liquid state
remains unsettled. To investigate the above, we present 133Cs
Nuclear Magnetic Resonance (NMR)measurements on Cs2CuCl4 at
temperature down to 50mK and applied magnetic field up to 15 T. We
first demonstrate that Cs NMR is an effective probe of the
magnetism in the compound through experiments in the paramagnetic
phase. Lower temperature NMR measurements as a function of the
strength and orientation of the applied magnetic field provide the
most complete picture of the different phases stabilized in
Cs2CuCl4. The magnetic character of these phases is discussed in
the context of the interplay between quantum fuctuations,
frustration and the Dzyaloshinskii-Moriya (DM) interaction.
Furthermore, we find evidence of three new phase transitions, as
well as an unexpected angular dependence of the saturation field.
These findings suggest that the accepted Hamiltonian requires
modifications, such as additional DM interactions. In order to
probe the nature of the spin liquid state, local magnetization and
spin-lattice relaxation rate measurements were performed. A
comparison with the result of a variational calculation using
Gutzwiller-projected mean-field theory demonstrates the 2D magnetic
behavior of the local magnetization. In addition, both temperature
and magnetic field dependence of the spin-lattice relaxation rate
suggests that the relevant low energy quasiparticle excitations
obey fermionic statistics.
Advisors/Committee Members: Mitrovic, Vesna (director), Marston, J. (reader), Valles, James (reader).

► Several aspects of transport in fractional quantum Hall (FQH) systems are investigated. The focus is on the FQH system at filling factor 5/2, which is…
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▼ Several aspects of transport in fractional quantum
Hall (FQH) systems are investigated. The focus is on the FQH system
at filling factor 5/2, which is expected to host non-Abelian
anyons.
We study interferometry signatures of the anyonic statistics
in the 5/2 state. It has been shown that non-Abelian states and the
Abelian 331 states may have identical signatures in Fabry-Perot
interferometry in the 5/2 FQH system. We study a Mach-Zehnder
interferometer and demonstrate that current and shot noise
measurements are able to distinguish the 331 states from the
non-Abelian counterparts. We find that the flux-dependent Fano
factor has the maximal value of 2*1.4e for the 331 state with
flavor symmetry. Without such symmetry, the Fano factor can reach
2*2.3e. On the other hand, for the Pfaffian and anti-Pfaffian
states the maximal Fano factor is 2*3.2e.
We also systematically study the transport properties of long
line junctions between a 5/2 FQH liquid and (i) another 5/2 liquid,
(ii) an integer quantum Hall liquid, and (iii) a quantum wire. We
study six candidates – two Abelian and four non-Abelian – for the
5/2 FQH effect. This momentum-resolved tunneling provides
information about the number, propagation directions, and other
features of the edge modes and thus helps distinguish several
competing models of the 5/2 state.
In the final section of the thesis, we derive a
fluctuation-dissipation theorem (FDT) for a general chiral system
in a non-equilibrium steady state. We consider a three-terminal
system with a chiral edge channel connecting the source and drain
terminals. We prove that the cross noise between the currents of
the chiral edge and the third terminal is related to the
differential conductance of the third terminal. The relation has
the same structure as the equilibrium FDT with the nonlinear
response in place of the linear conductance. The result can be
useful for detecting ''upstream'' modes on quantum Hall edges. We
also derive a FDT for heat and charge currents in multi-terminal
setups.
Advisors/Committee Members: Feldman, Dmitri (Director), Marston, John (Reader), Mitrovic, Vesna (Reader).

► The vortex matter in type-II superconductors continues to be a subject of great fascination in condensed matter physics. A longstanding theoretical and experimental problem is…
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▼ The vortex matter in type-II superconductors continues
to be a subject of great fascination in condensed matter physics. A
longstanding theoretical and experimental problem is the
identification of the ground state of the vortex lines in the
presence of quenched atomic disorder which acts as random pinning
centers. A possible edge contamination model has been proposed as a
mechanism behind the seemingly contradictory experimental results
for the ordered state of matter. This model could also explain the
lack of universality for the peak effect behavior exhibited in
samples with otherwise similar phase diagrams. Using a novel
neutron diffraction technique, we report structural evidence for
this edge contamination mechanism. This high-resolution method is
used to study the fine structure of the vortex matter in a niobium
crystal with a weak peak effect and a disordered zero-field-cooled
vortex matter. We find this disordered state is metastable and that
it can be restructured through a thermal cycling procedure. The
results are explained in a strained lattice framework. We then
perform Reverse Monte Carlo Refinements on our neutron scattering
data and the possible vortex structures for our crystal agree with
experimental results from an approach that combines spatial
information with reciprocal space scattering. Having confirmed the
existence of an edge contamination mechanism in this sample, we
oxidize the surface in order to reduce the impact of the
inhomogeneous surface barrier. By repeating our neutron diffraction
measurements, we find that oxidation process has smoothed the
magnetic field profile through the sample and improves the overall
structural order of the zero-field-cooled vortex matter. On the
other hand, the field-cooled vortex matter structure should be
independent of any edge contamination effect but surprisingly, this
scattering intensity in fact doubles after surface oxidation. This
result suggests that there is another source of disorder in the
niobium crystal that has been affected. We discuss our results in
the context of the peak effect and Bragg glass
models.
Advisors/Committee Members: Ling, Xinsheng (Director), Kosterlitz, James (Reader), Mitrovic, Vesna (Reader).

► Spin-orbit coupling (SOC) refers to the interaction between the orbital angular momentum and electron spin, which is typically a small perturbation in relative light elements.…
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▼ Spin-orbit coupling (SOC) refers to the interaction
between the orbital angular momentum and electron spin, which is
typically a small perturbation in relative light elements. However,
such interaction is proportional to the fourth order of the atomic
number and could be quite significant for heavy elements. Exotic
physics arising from significant SOC include the emergent phases in
Mott insulators with strong SOC and the exploded works on
topological insulators (TI). For the first research thread, the
combined effects of strong electronic correlations with SOC can
lead to a plethora of emergent novel quantum states. The second
research thread which also involves strong SOC and correlated
electrons lies at the intersection of topological insulator and
Kondo insulator. In this thesis, we first present an extensive
nuclear magnetic resonance (NMR) study of 23Na in Ba2NaOsO6, a
typical Mott insulator with SOC. We measured 23Na NMR spectra as a
function of temperature in different magnetic fields. The NMR
measurements reveals a paramagnetic at high temperatures, a broken
local point symmetry (BLPS) phase at intermediate temperatures and
a canted ferromagnetic phase at low temperatures. Most importantly,
we determine the field-temperature phase diagram of Ba2NaOsO6 which
shows that the intermediate BLPS phase extends to wider range in
high fields. Furthermore, by adopting numerical calculation, we
confirm that the breaking of local point symmetry is not due to
trivial intrinsic structure instability but is due to multipolar
interaction. Next, angle dependence of NMR spectra show that the
canted ferromagnetic order is suppressed along certain direction at
low temperatures. Secondly, we performed NMR measurements of
spin-lattice relaxation (T1) on 11B of Kondo insulator SmB6 at
ultra-low temperatures in various magnetic fields. We observed
constant low-energy density-of-states (DOS) which corresponds to
gapless excitation.
Advisors/Committee Members: Mitrovic, Vesna (Advisor), Valles, James (Reader), Marston, Brad (Reader).

▼ Heavy-fermion materials provide an exceptional setting
for the theoretical and experimental study of many intriguing
problems of modern condensed matter physics. Quantum criticality,
competition of Kondo and RKKY interactions, and unconventional
superconductivity are only a few of the many examples. One of the
most prominent representatives of this family of materials is
CeCoIn5, which, due to its rather unique properties, has attracted
intense research interest in recent years.In this thesis, we
present an extensive low temperature 115In nuclear magnetic
resonance (NMR) study of CeCoIn5, with a magnetic field applied
parallel to the â-axis of its tetragonal crystal. Our measurements
focus on the superconducting region of the H-T phase diagram,
exhibiting Tc ~2.3 K and Hc2 ~11.8 T. Through a consistent and
thorough analysis of the NMR observables, our investigation reveals
clear evidence of two separate phases within the superconducting
state and helps elucidate the details of their nature. In
particular, the first phase (referred to as LFSC), which occupies
most of the "volume" of the superconducting state, is found to be
well-described by a Ginzburg-Landau model for the vortex lattice in
an unconventional superconductor of dx2-y2 gap symmetry. The second
phase (referred to as HFLT), stabilized at a narrow high field, low
temperature corner of the phase diagram in the vicinity of Hc2,
shows signs of spatially modulated superconductivity. This is
interpreted as a likely realization of the elusive novel
superconducting phase theoretically predicted by Fulde, Ferrell,
Larkin, and Ovchinnikov in the 1960s. What is more, a long range
magnetic order is established within the limits of this phase,
associated with an incommensurate spin-density wave order. Lastly,
an additional region of distinct behavior is identified, lying in
between the LFSC and HFLT phases, which is possibly a true FFLO
state in the presence of strong antiferromagnetic
fluctuations.These results provide, as a whole, a remarkable
insight into unconventional superconductivity and quantum
magnetism, as well as their intricate interplay, not only in heavy
fermion materials but in condensed matter systems in
general.
Advisors/Committee Members: Mitrovic, Vesna (Director), Marston, John (Reader), Valles, James (Reader).

▼ We systematically studied the spin-dependent transport
and electrical noise characteristics of MgO-based magnetic tunnel
junctions (MTJs). Utilizing the coherent tunneling effect in the
MgO tunnel barrier, we have successfully fabricated MTJ devices
with large tunneling magnetoresistance (TMR) ratios. To explore the
magnetization properties of these devices at a fundamental level,
we first investigated the magnetization dynamics of sputtered CoFeB
thin films used as the free layers in MgO-based MTJ stacks by a
broad-band FMR spectrometer. Then we presented a new method for
estimating the magnetic anisotropy dispersion in MTJ arrays in
serial configurations, using the simulated field sensitivity maps.
Based on a revised Stoner-Wohlfarth model, we were able to assign
dispersion parameters to an actual MTJ array with arbitrary
magnetic attributes such as TMR ratio, coercivity and hysteresis
loops. As a result of our work, the field sensitivity of an MTJ
array was found to be inversely correlated to its anisotropy
dispersion and magnetic coercivity.
In the second part, we focused on the noise properties of MTJ
systems. At low frequencies, the flicker noise was measured in two
distinct MTJ arrays: MTJ Wheatstone bridges and MTJ discrete
resistors, each in their own serial arrays. For MTJ bridges, the
statistical dispersion in device resistance and normalized voltage
noise were attributed to the greater-than-expected magnetization
noise, whereas magnetic coupling among tightly-packed MTJ elements
was found to amplify the magnetic fluctuations in the MTJ discrete
resistors. Finally, high frequency shot noise measurements were
carried out to study the spin-dependent charge transport in MTJ
systems. The normalized shot noise, or Fano factor, exhibited a
sinusoidal-like variation with a continuous change in the relative
orientation between the magnetization vectors of MTJ free and
reference layers. We explained the noise behavior with a model of
sequential tunneling in the spin-blockade regime, in which the
faster transport of majority spin electrons in tunnel barriers is
modulated by the slower tunneling of minority spin
electrons.
Advisors/Committee Members: Xiao, Gang (Director), Mitrovic, Vesna (Reader), Dell'Antonio, Ian (Reader).

► In condensed matter physics the vortex lattice of type-II superconductors provides a prototype for studying the effect of random pinning on elastic systems. One important…
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▼ In condensed matter physics the vortex lattice of
type-II superconductors provides a prototype for studying the
effect of random pinning on elastic systems. One important problem
is to understand the structural transition from a theoretically
predicted ordered Bragg glass phase to a disordered vortex liquid
phase. Defects, such as screw dislocations, should play an
important role in mediating this order-disorder transition. It is
important to confirm the existence of these defect structures in
the vortex lattice. Experimentally it is difficult to probe the
detailed defect structures inside the vortex lattice. Traditional
methods can only provide information on the surface configuration
or an averaged picture of the bulk behavior of the vortex lattice.
Here we use a novel high-resolution neutron diffraction technique
to probe the angular orientation of the lattice planes as the flux
lines traverse the atomic crystal. Our results provide structural
evidence for screw dislocations inside the vortex lattice. The
anisotropic defect structure in the underlying atomic lattice
serves as a symmetry breaking field for the vortex lattice. The
strong dependence of the vortex lattice structure on the growth
procedure reveals that the system is metastable and can be
perturbed through thermal cycling to a possible ground state. We
measure the structure of the vortex lattice under different applied
magnetic fields and temperatures to study the interplay between
vortex-vortex interaction, vortex-atomic lattice interaction, and
thermal fluctuations. This high-resolution neutron diffraction
technique opens up a new way in studying the detailed structure of
the vortex lattice. Our results suggest that the vortex lattice in
low temperature superconductors with anisotropic defects in the
atomic lattice could be an excellent candidate for exploring the
entangled flux liquid phase.
Advisors/Committee Members: Ling, Xinsheng (Director), Kosterlitz, J. (Reader), Mitrovic, Vesna (Reader).

▼ Fractional quantum Hall (FQH) liquids are interesting
two-dimensional electron systems that possess quasiparticle
excitations with fractional charges, obeying quantum statistics
different from those of bosons and fermions. In particular, the FQH
liquid at filling factor 5/2 was proposed to host Majorana bound
state (MBS) with exotic non-Abelian statistics. A collection of
MBSs can span a topological Hilbert space, in which each
many-particle state is topologically distinct, depending on the
historical trajectories of all the MBSs in the system. Logic
operations in quantum computation can be encoded in the linear
transformations in topological Hilbert space and in principle be
protected against local defects and perturbations, which are
topologically trivial and cannot induce transitions between
different many-particle states. Despite such intriguing theoretical
picture, experiments probing the nature of the 5/2 FQH liquid are
controversial. In this dissertation, we provide an explanation of
two seemingly contradicting experiments in the 5/2 FQH liquid, by
exploring the role of electrostatic interaction closely related to
the geometries of the devices. We also construct several new 5/2
FQH states, by making use of the particle-hole symmetry in FQH
systems, to account for a recent experiment observing upstream
neutral edge transport in the 5/2 FQH liquid, which ruled out most
of the existing theories. In addition to the new particle-hole
states, we propose another topological description of the 5/2 FQH
liquid which reconciles all existing transport experiments. Later,
we turn our attention to the MBSs in superconductor systems. We
study the approaches to minimizing the decoherence of a
Majorana-fermion-based qbit due to its interaction with
environment, based on a full classification of the fermionic zero
modes in a system of interacting Majorana fermions.
Advisors/Committee Members: Feldman, Dmitri (Director), Kosterlitz, John (Reader), Mitrovic, Vesna (Reader).

► The superconductor-insulator transition (SIT) is a well-known phenomenon that is observed in many types of superconducting systems, including elemental films and high Tc superconductors. Ultrathin…
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▼ The superconductor-insulator transition (SIT) is a
well-known phenomenon that is observed in many types of
superconducting systems, including elemental films and high Tc
superconductors. Ultrathin films provide a versatile platform from
which to study this ubiquitous, and possibly universal, transition.
Two different theoretical approaches have been made to describe the
SIT in thin films. The first predicts that Cooper pairs (CPs) exist
in the insulating phase, and the second proposes that the
transition is driven by pair-breaking, yielding an insulator of
localized single electrons.
Recent experiments showing the existence of an insulating
phase of localized CPs have provided significant support for the
first mechanism. This CP insulating (CPI) phase exhibits dramatic
transport features including a giant magnetoresistance (MR) peak.
Previous work in the Valles lab found such a phase in ultrathin
amorphous Bi films quench-condensed onto a substrate with a
nanohoneycomb (NHC) hole array. The local Cooper pairing
correlations of the CPI phase were evident from Little-Parks-like
MR oscillations due to the hole array.
This thesis describes investigations of the mechanism for CP
localization and the emergence of the CPI phase in NHC films. There
are three main results. First, the CPI phase appears to be induced
in NHC films by the undulating surface of the substrate, which
produces regular spatial variations in thickness that give rise to
nanoscale CP islands. Second, the giant positive MR characteristic
of the CPI phase emerges with increasing film thickness
simultaneously with the MR oscillation signal, associated with
local CP phase coherence. The insulating phases of the thinnest
films and films in magnetic fields just beyond the MR peak likely
consist of CPs totally localized to their islands. Third, amorphous
holey films of uniform thickness do not exhibit a CPI phase. These
films go from superconductors directly to fermionic insulators with
decreasing thickness. These results lend further insight to the
nature of the CPI phase and also require at least two classes of
SITs: one that can describe the presence of a CPI phase and one
that can describe a transition from a superconductor to a fermionic
insulator.
Advisors/Committee Members: Valles, James, Jr. (Director), Mitrovic, Vesna (Reader), Feldman, Dmitri (Reader).

▼ FeSb2 is a narrow band semiconductor, whose magnetic
susceptibility strongly resembles that of FeSi, an archetype
example of Kondo insulator. The unusual spectral weight transfer in
the optical conductivity measurement and ab initio LDA+U
calculations imply that FeSb2 is an unconventional semiconductor
with strong electron correlations. In order to study the anisotropy
and probe the electronic structure of FeSb2, single crystals are
grown by high temperature flux method and carriers are introduced
into FeSb2 by substituting Fe and Sb with Co/Cr and Te
respectively. We investigated the magnetic and electrical
properties of FeSb2 and the doping effects. Hall measurement on
FeSb2 reveals that multiple band carriers with very large mobility
difference are involved in the electrical transport. This can be
explained by the orbital selective hybridization of the Fe
d-orbitals with Sb-p electrons in FeSb2 involving only one
electronic subsystem. Band structure calculation suggests that
FeSb2 is close to a magnetic instability. Indeed, various magnetic
orders are experimentally observed in doped FeSb2. For example, the
activated Pauli paramagnetism of FeSb2 evolves into a weak
ferromagnetism in Fe1-xCoxSb2 and a complex canted
antiferromagnetic structure in Fe1-xCrxSb2 and Fe(Sb1-xTex)2.
Metallic states are readily induced by doping FeSb2 as a result of
band filling. Positive colossal magnetoresistance mainly attributed
to quasi 1D weak localization is observed in Co doped FeSb2. With
Te substitution, FeSb2 evolves from a strongly correlated
semiconductor into a moderate heavy fermion metal. Most of the
Kondo insulator materials are cubic, with the exception of CeRhSb
and CeNiSn, and 4f intermetallics. An important question is that if
the Kondo insulator scenario can be applied to 3d materials, e.g.
FeSi and FeSb2. FeSb2 can be obtained in single crystal form with
high quality and can be readily doped with carriers, thus it
represents a model system with only 3d elements to study this
physics. Our results indicate that the underling physics of FeSb2
can be well described by the Kondo insulator picture and FeSb2
resembles FeSi in many ways as a prototypical example of a
nonrare-earth containing Kondo insulator.
Advisors/Committee Members: Mitrovic, Vesna (director), Petrovic, Cedomir (director), Marston, Brad (reader), Valles, James (reader).

► Amorphous oxide semiconductors (AOSs) have been intensively studied during the last 15 years due to their superior properties. The major application of AOS thin film…
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▼ Amorphous oxide semiconductors (AOSs) have been
intensively studied during the last 15 years due to their superior
properties. The major application of AOS thin film transistors
(TFTs) is displays. Compared to the previous generation TFT
material—amorphous silicon (a-Si)—AOSs have higher mobility, and
can be easily deposited by various deposition techniques at room
temperature on arbitrary substrates. This thesis focuses on the
characteristics and stability of a specific AOS, amorphous indium
zinc oxide (a-IZO), as an electronic material for TFTs. We have
fabricated a-IZO TFTs with a top-gate structure by using the
in-situ gate dielectric formation technique: some metals—such as Al
and Hf—can react with IZO to form an oxide insulator layer in the
absence of kinetic constraints. The in-situ dielectric formation
can provide a high-quality insulator/IZO interface with a low
interface trap density, leading to excellent TFT performance. The
subthreshold slope in our best IZO TFTs reaches a value of 62
mV/decade, close to the room temperature theoretical limit, and our
devices also show on/off current ratios above 107 and a high
extracted field effect mobility around 100 cm2/V·s. These are
state-of-the-art results in AOS TFTs. Another focus in our work is
the stability of IZO TFTs, where oxygen vacancy generation and
migration are used to explain the observed experimental threshold
voltage shifts in a-IZO TFTs as a function of aging and applied
gate bias. As a possible future research direction, the high
electron mobility and controllable high electron density make a-IZO
a promising candidate for high-speed, high-power radio frequency
transistors. We propose a vertical transistor structure with buried
multi-gate fingers. Unlike in the previously studied single-crystal
nanowire or permeable base vertical transistors, the properties of
a-IZO make it possible to regrow the vertical semiconductor
channels without any high temperature epitaxy, lowering fabrication
complexity and cost while making the vertical IZO device compatible
with arbitrary substrates. Preliminary fabrication process and
characterization results of the vertical-current-flow a-IZO TFT are
shown and discussed.
Advisors/Committee Members: Zaslavsky, Alexander (Advisor), Paine, David (Advisor), Valles, James (Reader), Mitrovic, Vesna (Reader).

► The nematic phase transition in Fe-based superconductors (FeSCs) has been a topic under intensive investigation. So far it is commonly accepted that the structural transition…
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▼ The nematic phase transition in Fe-based
superconductors (FeSCs) has been a topic under intensive
investigation. So far it is commonly accepted that the structural
transition from tetragonal (C4) to orthorhombic (C2) symmetry in
FeSCs has an electronic nematic origin due to the unusual
anisotropy in resistivity, optical conductivity and orbital
occupancy observed above the structural transition. However, recent
studies of (Ba, Eu)Fe2(As1-xPx)2 by magnetic torque measurements
show the existence of a “true” nematic transition well above the
commonly accepted structural/nematic transition .Controversies
about this “true” nematic phase transition arise as residue strains
or external applied fields are known to break C4 symmetry and
render the structural transition merely a crossover. We performed
high resolution AC micro-calorimetry and SQUID magnetometry
measurements of BaFe2(As1-xPx)2 (x=0, 0.3) to investigate the
various phase transitions and to explore the “true” nematic phase
transition. The advantageous design of our membrane calorimeter
allows us to perform high resolution studies of the thermodynamic
phase transitions without any symmetry breaking fields. Our results
suggest that there is not a second order “true” nematic phase
transition in BaFe2(As1-xPx)2 even though the Ginzburg-Landau model
used to fit the magnetic torque data indicates that the expected
thermal anomaly should be within our experimental resolution. In
addition to the above, we present specific heat and magnetization
studies of Ba1-xNaxFe2As2 in search of the recent discovered
emergent reentrant C2 to C4 symmetry SDW transition in this series
of compound. Our results indeed locate a new phase transition in
Ba0.74Na0.26Fe2As2 at 45K. However, the absence of the conventional
SDW transition at around 80K in our data leaves doubt about the
exact nature of this new phase transition. We also systematically
studied the effects of heavy ion irradiation (HII) on the
anisotropy of YBCO single crystals by angular rotation specific
heat measurements. We found that the anisotropy of YBCO decreases
by approximately a factor of two with an irradiation dose of 6T
(matching field). The dependence of anisotropy on irradiation doses
agrees well with the prediction from a simple phenomenological
model that takes into account the anisotropic scattering caused by
columnar defects created in HII.
Advisors/Committee Members: Ling, Xinsheng (Director), Welp, Ulrich (Director), Marston, John (Reader), Mitrovic, Vesna (Reader).

► Particles other than bosons and fermions can exist in two dimensions. One possibility is that when one particle makes a circle around another particle the…
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▼ Particles other than bosons and fermions can exist in
two dimensions. One possibility is that when one particle makes a
circle around another particle the total many-particle wave
function acquires a non-trivial phase factor. Such particles are
called Abelian anyons. In a more exotic situation, the action of
moving one particle around other particles or in other words,
particle braiding, is represented by a unitary matrix acting on the
quantum-state vector. If the braiding matrices do not commute with
each other, the particles are called non-Abelian anyons.\ The
existence of both Abelian and non-Abelian anyons has been predicted
in Fractional Quantum Hall systems. Quasiparticles in the Laughlin
states with filling factor ν=1/(2m+1) are Abelian anyons. On
the other hand, quasiparticles in the Moore-Read and Read-Rezayi
states with filling factors ν=5/2 and ν=12/5 ,
respectively, are proposed to be non-Abelian anyons. However, no
experimental observation of the exchange statistics of identical
anyons has been reported so far. \ In this thesis we demonstrate
that the current and shot noise in the topologically nontrivially
set-up of the electronic Mach-Zehnder interferometer can be used to
detect anyonic statistics. The transport is not sensitive to the
fluctuations of the topological charge inside the interferometer
and hence the interference picture is not destroyed by the
tunneling of low-energy neutral excitations between the edges and
localized states in the interferometer. The current and noise
exhibit non-Analytic dependences on small tunneling amplitudes. The
low-temperature Fano factor is always below 1 for Abelian anyons
and can greatly exceed 1 for non-Abelian statistics.
Advisors/Committee Members: Feldman, Dima (director), Mitrovic, Vesna (reader), Ying, See-Chen (reader).